1. Field of the Invention
This invention relates to the preparation of transparent iron oxide pigment dispersions. The dispersions are prepared by milling a mixture of iron oxide pigment and liquid vehicle in a bead mill containing glass beads as the grinding media.
2. Description of the Prior Art
Transparent iron oxide pigment dispersions have traditionally been prepared by a modified flushing process. Flushing refers to the direct transfer of pigment particles from an aqueous to a nonaqueous phase. The aqueous phase is mixed with a nonaqueous liquid vehicle with heavy-duty mixers. The water separates from the system and is removed, leaving the pigment dispersed in the liquid vehicle.
This process is rather cumbersome and labor intensive. The resulting dispersion must be thinned, drum settled, filtered and standardized. Moreover, this process is a batch process and much of the iron oxide pigment may be wasted in this process.
Other techniques are known in the art for dispersing pigments into liquid vehicles. The most pertinent technique involves the milling of a pigment-vehicle mixture in a bead mill which contains, as the grinding media, glass beads having small diameters.
Although this technique has been used for making various pigment dispersions, to applicants' knowledge, it has not been used for making transparent iron oxide pigment dispersions. There are several possible reasons for this. In order to obtain transparent iron oxide pigment dispersions, the average particle size of the pigment must be less than 0.1 micron in size. Unless the average particle size of the pigment is below this size, the pigment is unable to transmit visible light which has wavelengths from 400 to 700 millimicrons. It is difficult to find grinding media which will produce iron oxide pigment dispersions with particles of this size. It is also difficult to find grinding media which are strong enough to withstand the contact with iron oxide pigment which is more abrasive than most pigments.